Method and system for adaptive channel access in unlicensed spectrum

ABSTRACT

A method and a device for adaptive channel access are disclosed. In an embodiment includes adaptively adjusting, by a small base station (SBS), access parameters for small cells to ensure quality of service (QoS) to cellular users while minimizing collision probability for WiFi users.

This application claims the benefit of U.S. Provisional Application No.62/140,972, filed on Mar. 31, 2015, which application is herebyincorporated herein by reference

TECHNICAL FIELD

The present invention relates to a method and system for wirelesscommunications, and, in particular embodiments, to a method and systemfor adaptive listen-before-talk (LBT) channel access for unlicensedspectrum.

BACKGROUND

It is predicted that mobile data usage will increase by 11-fold by 2018.To deal with this challenge, many new techniques have been proposed toimprove data rate in the fifth generation (5G) wireless communicationsystems. However, the scarcity of the licensed spectrum for cellularnetworks is still the main bottleneck for further improvement of datarate. As a result, exploiting the unlicensed bands in small cells,currently used by WiFi, becomes a promising option.

WiFi is the most popular and successful technology to provide wirelessservice on unlicensed bands in a local area. With low cost and high datarate, WiFi systems already are the dominant player on all unlicensedbands in 2.4 GHz and 5 GHz. Most consumer electronic devices now comewith a WiFi module. However, its spectrum efficiency is low when thenetwork is overloaded.

SUMMARY

An embodiment method for adaptive channel access includes a small basestation (SBS) adaptively adjusting access parameters for small cells toensure quality of service (QoS) to cellular users while minimizingcollision probability for WiFi users.

An embodiment method for adaptive channel access includes adaptivelyadjusting, by a small base station (SBS), access parameters for smallcells to ensure quality of service (QoS) to cellular users whileminimizing a collision probability to WiFi users, wherein minimizing thecollision probability to the WiFi users comprises keeping or decreasingan optimal backoff window size when the collision probability to theWiFi users is below a threshold value and increasing the optimal backoffwindow size when the collision probability to the WiFi users is abovethe threshold value.

An embodiment method for adaptive channel access includes determining,by a small base station (SBS), a fraction of SBS air time share on anunlicensed band for cellular users to ensure quality of service (QoS)for the cellular users, estimating, by the SBS, a number of WiFi userson the unlicensed band, adjusting, by the SBS, the fraction of SBS airtime share on the unlicensed band for the cellular users when acollision probability to the WiFi users is above a threshold value andnot adjusting, by the SBS, the fraction of SBS air time share on theunlicensed band for the cellular users when the collision probability tothe WiFi users is below the threshold value.

An embodiment SBS includes a processor configured to adaptively adjustaccess parameters for small cells to ensure QoS to cellular users whileminimizing collision probability for WiFi users.

An embodiment small base station (SBS) comprises a processor and anon-transitory computer readable storing medium storing programming forexecution by the processor, the programming including instruction toadaptively adjust access parameters for a small cell to ensure qualityof service (QoS) to cellular users while minimizing a collisionprobability to WiFi users, wherein minimizing the collision probabilityto the WiFi users comprises keep or decrease an optimal backoff windowsize when the collision probability to the WiFi users is below athreshold value and to increase the optimal backoff window size when thecollision probability to the WiFi users is above the threshold value.

An embodiment SBS includes a processor and a non-transitory computerreadable storage medium storing programming for execution by theprocessor. The programming includes instructions for initializing amaximum fraction of SBS air time shared on an unlicensed spectrumaccording to a WiFi traffic estimate, determining an amount of availablelicensed spectrum does not guarantee a quality of service (QoS) to smallcell users, determining power and rate optimization to minimize acollision probability of WiFi users, and determining an SBS backoffwindow size.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawing, in which:

FIG. 1 illustrates a system model;

FIG. 2 illustrates an adaptive channel access scheme according toembodiments of the invention;

FIG. 3 illustrates an adaptive channel access scheme according toembodiments of the invention;

FIG. 4 illustrates an adaptive channel access scheme according toembodiments of the invention;

FIG. 5 illustrates an adaptive channel access scheme according toembodiments of the invention;

FIG. 6 illustrates the fractions of the unlicensed band occupied versusthe number of served cellular users for different licensed spectrumbandwidths;

FIG. 7 illustrates an increase of collision probability when the numberof cellular users increases;

FIG. 8 illustrates an admission control scheme for the unlicensed band;

FIG. 9 illustrates the throughput of a number of cellular usersaccording to different access schemes; and

FIG. 10 illustrates a computing device for the SBS according to anembodiment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The structure, manufacture and use of the presently preferredembodiments are discussed in detail below. It should be appreciated,however, that the present invention provides many applicable inventiveconcepts that can be embodied in a wide variety of specific contexts.The specific embodiments discussed are merely illustrative of specificways to make and use the invention, and do not limit the scope of theinvention.

Listen-before-talk (LBT) regulations in the unlicensed spectrum (e.g.,5G unlicensed (5G-U) spectrum) require equipment to check for potentialoccupants of the unlicensed spectrum, i.e., listen, before transmitting,i.e., talk. LBT is a mandated requirement for using the unlicensedspectrum in some regions. Moreover, LBT has been adopted as a workingassumption for licensed assisted access (LAA) study item in release 13of LTE-A. The LBT-compatible access method may be unfair to WiFi userswhen the number of cellular users increases. In addition, existing LBTaccess protocols, such as 802.11 distributed coordination function(DCF), are fixed and therefore non-adaptive. That is, they are unable toprovide on-demand access to the unlicensed spectrum depending on theavailability of licensed spectrum resources, to guarantee quality ofservice (QoS) for the cellular users, or to ensure that the collisionprobability of the WiFi users is below a certain threshold.

With these channel selection access methods the unlicensed band may beinefficiently used.

In markets without LBT requirements, 5G mobile operators can adopt aduty cycle approach to share the spectrum fairly with other unlicensedspectrum incumbents such as WiFi. In the duty cycle approach, the smallbase station (SBS) controls the utilization of unlicensed bands.

Embodiments of the invention enable a LTE (U-LTE) SBS to access theunlicensed bands. Embodiments provide a fair coexistence betweencellular users and WiFi users on the unlicensed bands and the joint useof the licensed and unlicensed bands for the cellular users in the SBS.

Further embodiments of the invention provide an adaptive channel accessscheme for small cells to share the unlicensed spectrum with WiFi accesspoints (APs). In embodiments, the method adaptively adjusts accessparameters for the small cells to ensure QoS for cellular users whileminimizing collision probability for WiFi users.

According to embodiments, the adaptive LBT channel access scheme allowsthe cellular users to access the unlicensed spectrum. In embodimentsaccess probability parameters are adaptively adjusted to minimizecollision probability to the WiFi system. The parameters may be the WiFitraffic in the unlicensed band(s), available licensed bandwidth, and theQoS requirements of the cellular users. These parameters are alsojointly considered to provide and adjust if necessary a minimum backoffwindow size for the unlicensed spectrum at the small cell. Embodimentsrelate to integrated spectrum access with dual operation over both thelicensed and unlicensed spectrums.

FIG. 1 shows several WiFi access points (WiFi APs) in the coverage of asingle small base station (SBS). The SBS can support both licensed andthe unlicensed bands to support (uplink, downlink or uplink anddownlink) transmission(s) to the cellular users. The small base stationmay support a small cell. A small cell may be a low-powered radio accessnode that operates in a licensed and an unlicensed spectrum and thattypically has a range of several meters to 1 or 2 kilometers. The smallcell is “small” compared to a mobile macro cell (supported by a macrocell base station), which may have a range of a few tens of kilometers.

In some embodiments small cells may encompass femtocells, picocells ormicrocells. In other embodiments small cells provide a small radiofootprint, which can range from 10 meters within an urban or inin-building locations to 2 km for a rural location. Picocells andmicrocells may have a range of a few hundred meters to a few kilometers.In various embodiments the small cell may comprise a Home eNode B (HeNB)as an LTE femtocell.

The some embodiments the small cell base station has a maximum transmitpower of up 30 dBm in the unlicensed band.

The WiFi AP(s) may only use the unlicensed bands to serve the WiFi usersand different WiFi APs may use different unlicensed bands to avoidstrong co-channel interference. The number of cellular users in the SBSand WiFi APs is N, n_(k) for each WiFi AP K.

An adaptive channel access scheme 200 according to an embodiment isillustrated in the flow chart of FIG. 2. Here, the backoff window size Wof the SBS for the unlicensed band is adaptively adjusted based on WiFitraffic (traffic load), QoS requirements of the cellular users (e.g.,minimum rate requirement for small cell users) and the availablelicensed bandwidth. In embodiments the backoff window size W may be aminimum backoff window size. The backoff window size W is not fixed asin conventional systems. As will be discussed below, when there isenough achievable licensed bandwidth to serve the cellular users the SBSwill not access the unlicensed band(s). In contrast, when there is notenough available licensed bandwidth the SBS will access the unlicensedband(s). The access to the unlicensed band(s) may be restricted for thecellular users when the collision probability for the WiFi users passesa certain threshold and is too high.

The available licensed bandwidth and the available unlicensed bands arejointly allocated to improve the overall spectrum efficiency of thecellular system and to minimize the impact on the WiFi system. Eventhough the collision probability is minimized, it may still exceed athreshold value which WiFi APs can tolerate due to the large number ofcellular users. The access to the unlicensed bands may be balanced by anadmission control scheme in order to provide fair utilization of theunlicensed bands for the WiFi users.

The process starts with processing block 204. In processing block 204,the SBS determines the amount of available licensed band C.

Then, in decision block 206, the SBS judges whether the availablelicensed band C is enough to serve all cellular users. If the availablelicensed band C is enough to guarantee QoS to serve all present cellularusers then the SBS backoff window size W is set to infinity atprocessing block 208. The process returns to processing block 204 wherethe SBS determines again the amount of available licensed band C. Ifthere is not enough licensed band available, the process advances toprocessing block 210. In this scenario the available licensed spectrumdoes not guarantee QoS to all cellular users.

At processing block 210 the SBS determines optimal parameter values tominimize the collision probability to WiFi users. This is equivalent tominimize the impact brought by the SBS on the unlicensed band(s). Forexample, the optimization problem can be described as minimizing thetotal amount of air time share required by the cellular system on theunlicensed spectrum in order to satisfy the QoS for the cellular users.

In some embodiments the minimum impact of the SBS can be formulated as:

$\begin{matrix}{{\underset{\{{\{{c_{i}^{(l)},s_{i}^{(u)},p_{i}^{(l)}}\}}_{i \in U}\}}{\min\;}{\sum\limits_{i \in U}S_{i}^{(u)}}}{wherein}{{R_{i} \geq r_{i}},{{\forall{i \in U}};}}} & ({Equation}\;)\end{matrix}$

is the data rate requirement for each cellular user i. The data rate maybe guaranteed in every time slot t, wherein r_(i) is the minimum datarate for the cellular user i. In some embodiments the minimum data rateensures the QoS requirements of the cellular user or, in other words,the QoS requirement corresponds to the minimum data rate or is the sameas he minimum data rate. The equation further requires that thebandwidth allocation is smaller than C_(L) and fulfills

${{\sum\limits_{i \in U}\; c_{i}^{(l)}} \leq C_{L}},$

and that the transmit power is constrained on the licensed bandaccording to the following equation:

${{\sum\limits_{i \in U}\; p_{i}^{(l)}} \leq P_{T}^{(l)}},$for all p_(i) ^((I)) greater or equal to 0, c_(i) ^((I)) greater orequal to 0, s_(i) ^((u)) greater or equal to 0 and for all cellularusers i element of U.

The Equation minimizes the collision probability experienced by WiFiusers at each time slot t. The channel collision probability of the WiFiusers monotonically increases with the channel access probability of theSBS. Therefore, higher channel access probability incurs highercollision probability. On the contrary, the available unlicensedbandwidth at the SBS monotonically increases with the channel accessprobability. Accordingly, minimizing the collision probability to WiFiusers is equivalent to minimizing the required unlicensed band at theSBS.

The SBS may determine the optimal minimum amount of bandwidth needed toprovide QoS to the cellular users based on the transmission power p_(i)^((I)) allocated on the licensed band for the cellular users i, thelicensed bandwidth c_(i) ^((I)) allocated to the cellular user i and thetime fraction s_(i) ^((u)) allocated to the cellular user i on theunlicensed band for the WiFi AP. In some embodiments, the optimal valuescan be found by minimizing the sum for the time fraction s^((u)) _((i))for all cellular users i.

In some embodiments, the optimal transmission power p_(i) ^((I)) and theoptimal fraction of the licensed band c_(i) ^((I)) can be determined byapplying Lagrangian multipliers (e.g., conventional Lagrangianmultipliers) based on Karush-Kuhn Tucker (KKT) conditions to theEquation.

After determining the optimal amounts of air time share value s_(i)^((u)), transmit power p_(i) ^((I)) and bandwidth allocation to thelicensed band c_(i) ^((I)), the process advances to processing block212. Here the SBS determines the current fraction of SBS air times onthe unlicensed spectrum, S^((u))(t)(e.g., for all cellular users). Ifthe Equation is 0 (i.e., S_(i) ^((u))=0) no air time share S is neededon the unlicensed band. In other words, the cellular users can be servedexclusively on the licensed band. If the Equation is greater than 0, airtime share S on the unlicensed band is required to satisfy the QoS forthe cellular users.

In the next process block, block 214, the SBS estimates the number ofWiFi users on the unlicensed spectrum. The number of WiFi users on theunlicensed band may be estimated by applying a Kalman filteringestimation method.

In the next process block, block 216, the SBS determines the minimumbackoff window size W based on the optimal factors for p_(i) ^((I)),c_(i) ^((I)), s_(i) ^((u)) and S^((u))(t), and an estimation of a numberof WiFi users on the unlicensed band.

Based on the optimal factors, the collision probability p^((WF)) of theWiFi users on the unlicensed band and the channel access probability ofthe SBS η^((BS)) is calculated based onS ^((u))(t)=η^((BS))(t)1−τ(t))^(n)wherein τ is the stationary probability that the n WiFi users served bythe WiFi AP transmit a packet. Substituting the determined optimizedrequired band on the unlicensed band S^((u)) in this equation arelationship between τ and η^((BS)) can be obtained.

With this relation the following equations become two nonlinearequations with two unknown variables τ and p^((WF)):

$T_{k} = \frac{2 \times \left( {1 - {2p_{k}^{({WF})}}} \right)}{{\left( {1 - {2p_{k}^{({WF})}}} \right)\left( {W_{k}^{({WF})} + 1} \right)} + {p_{k}^{({WF})}{W_{k}^{({WF})}\left( {1 - \left( {2p_{k}^{({WF})}} \right)^{m_{k}^{({WF})}}} \right)}}}$wherein W_(k) ^((WF)) is the backoff off window size of the WiFi AP kand m_(k) ^((WF)) is the maximum contention stage for the WiFi AP. Thecollision probability p_(k) ^((WF)) for the WiFi AP k is:p _(k) ^((WF))=1−(1−η_(k) ^((BS)))(1−τ_(k))^(n) ^(k) ⁻¹ ,∀k∈{1, . . .,K}p _(k) ^((BS))−1−(1−τ_(k))^(n) ^(k) ,∀k∈{1, . . . ,K},

Accordingly, the variables τ, p^((WF)) and η^((BS)) can be calculated.Finally, the minimum backoff window size W (e.g., W_(k) ^((BS))) can bederived from the channel access probability η^((BS)) of the SBS based onthe following formula:

${\eta_{k}^{({BS})}\left( W_{k}^{({BS})} \right)} = \frac{2\left( {1 - {2p_{k}^{({BS})}}} \right)}{\left( {1 - {2p_{k}^{({BS})}}} \right)\left( {W_{k}^{({BS})} + 1} \right)}$

The minimum backoff window size is in a range between 20 ms and 640 msin some embodiments. In other embodiments the backoff window size W isbetween 100 ms and 300 ms. In yet other embodiments any suitable timerange may be used.

Then the process advances to decision block 218. In decision bock 218the SBS determines whether or not the collision probability to the WiFiusers is below a threshold value. If the collision probability is belowthe threshold value the SBS uses the determined fraction of SBS air timeshare S on the unlicensed band. Therefore, the SBS keeps or reduces theminimum backoff window size W as set in processing block 216. Theprocess then moves back to decision block 206 and evaluates againwhether or not the licensed band provides enough bandwidth to supportthe present cellular users. Based on the decision in the decision block206 the process repeats or iterates the steps 206/208/204 or 206-214.

If the collision probability is above the threshold value the SBS cannotutilize the determined fraction of SBS air time S. The SBS air timeshare S on the unlicensed band must be reduced. In order to achievethis, the process advances to processing block 220. Here, the SBSdecreases the fraction of SBS air time S on the unlicensed band. Thismeans that the SBS has to perform admission control on the cellularusers. Accordingly, not all cellular users can be serviced by the SBS.In other words, not all cellular users who request admission to thelicensed and unlicensed bands will be admitted to these bands. Some ofthe cellular users will be rejected so that the collision probabilityfor the WiFi users is kept at a certain level. By rejecting admission tosome cellular users the air time share S on the unlicensed band of thecellular users is reduced.

By increasing the backoff window size (e.g., minimum backoff windowsize), the SBS is less aggressive in accessing the unlicensed spectrumand by decreasing the backoff window size (e.g., minimum backoff windowsize), the SBS is more aggressive. Accordingly, the SBS increases theminimum backoff window size when the collision probability to the WiFiusers is above the threshold value and the SBS keeps or reduces theminimum backoff window size when the collision probability of the WiFiusers is below the threshold.

Moreover, the minimum backoff window size W may determine the SBS airtime fraction S on the unlicensed band. The more aggressive the minimumbackoff window size W is set the more air time share is used on theunlicensed band. If the minimum backoff window size is increased, lessSBS air time share can be utilized.

The process advances to process block 222. The SBS determines an optimalset of cellular users (based on all cellular users requesting admissionto the system). The process moves back to process block 214 and iteratessteps 214-218. The number of cellular users who are admitted to the SBSmay be optimized so that a maximum number of cellular users are admittedto the SBS.

In some embodiments the SBS may exploit not only one unlicensed band butmore than one unlicensed bands. For example, the SBS may exploit aplurality of unlicensed bands via a plurality of WiFi APs in order tominimize the number of unserved cellular users. In other words, the moreunlicensed bands are available the more cellular users can be servedwithout (substantially) impacting the WiFi users at the different WiFiAPs.

In some embodiments, the method provides a solution to balance thecollision probability among the plurality of WiFi APs. Accordingly, theoptimization can be described as minimizing the maximum collisionprobability among all WiFi APs.

An adaptive channel access scheme 300 according to an embodiment isillustrated in the flow chart of FIG. 3. Here, the SBS uses differentunlicensed bands of different WiFi APs. The minimum backoff window sizeW is calculated and the collision probability is determined for eachunlicensed band.

The channel access scheme is similar to that of the embodiment describein FIG. 2. The SBS determines the amount of available licensed band C ina first process step 304. The SBS then determines whether or not C isenough to guaranteed QoS for all cellular users 306. If it does the SBSsets the minimum backoff window sizes W_(k) for each WiFi AP to infinityat 308. If not the SBS determines the optimal parameter values tominimize the collision probability to WiFi users in all unlicensed bandsat 310.

This can be described as

${\min\limits_{{\{{c_{i}^{(l)},s_{k,i}^{(u)},p_{i}^{(l)}}\}}_{{i \in \mathcal{U}},{k \in {\{{1,\ldots\mspace{14mu},K}\}}}}}{\max\limits_{k}\left\{ {{\eta_{1}{\sum\limits_{i \in \mathcal{U}}s_{1,i}^{(u)}}},\ldots\mspace{14mu},{\eta_{K}{\sum\limits_{i \in \mathcal{U}}s_{K,i}^{(u)}}}} \right\}}},$

wherein η_(k) (for all k element {1, . . . , K}) is the weighted factoron the utilization of the licensed bands from different WiFi APs. Then,at 312, the SBS determines the maximum fraction of SBS air time S_(k)^((u)) for each WiFi AP k and the weighted factor η_(k) can becalculated according to η_(k)=S^((u)) _(k)/Σk∈{1, . . . , K}S^((u))_(k). Afterwards, at process blocks 314 and 316, the SBS estimates thenumber of WiFi users on the unlicensed bands and the minimum backoffwindow size W_(k) for each WiFi AP. If collision probability to WiFiusers is below a threshold for each WiFi AP the process advances todecision block 306 to iteratively start from there again. If thecollision probability to WiFi users is above a threshold for at leastone WiFi AP, the process advances to processing block 320. This meansthe if the collision probability to any WiFi AP is over the threshold,then the collision probability to the remaining WiFi APs has alsocrossed the threshold due to the balanced property. The SBS makes thisdecision in decision block 318.

When the collision probability to WiFi users is above the threshold forat least one WiFi AP the SBS decreases the fraction of SBS air timeshare S on the at least one unlicensed band. For example, when thecollision probabilities at two WiFi APs are over the threshold valuethen the collision probability at the remaining WiFi APs is alsoconsidered to be over the threshold value and SBS needs to decrease thefraction of SBS air time. The process then advances to process step 322to determine an optimal set of cellular users to be served by the givenavailable licensed band C and the fraction of the air time share S onthe unlicensed bands. The process then in turn reiterates the processsteps beginning at process block 314. In alternative embodiments the SBSonly adjusts the fractions S^((u)) _(k) of the two WiFi APs that areover the threshold value while the S^((u)) _(k) remain unadjusted.

In some embodiment, if the threshold value is violated for only a fewWiFi APs of a large plurality of WiFi APs the minimum backoff windowsize for these few WiFi APs can be increased by an estimated amount orby a set amount without going through the iteration and the calculationagain. In some embodiments, the backoff window size for the few WiFi APscan be set to infinity.

The threshold values for the WiFi APs can be the same for all WiFi APSor can be different for some or all of them.

In some embodiments the cellular users to be selected should be themaximum number of cellular users served by the SBS.

${\max\limits_{{{\{{I_{i},c_{i}^{(l)},s_{k,i}^{(u)},p_{i}^{(l)}}\}}i} \in \mathcal{U}}{\sum\limits_{i \in \mathcal{U}}^{\;}\; I_{i}}},$

wherein I_(i) is an integer variable, when cellular user i is selected,I_(i)=1 or otherwise 0. This guarantees that the data rate of theselected cellular users is not less than requested.

FIG. 4 shows an adaptive channel access scheme 400 for at least one WiFiAP according to an embodiment. Here, the process starts again bydetermining the availability of the licensed band C, 404. If there isenough available licensed band C the SBS does not use the unlicensedband(s) because the licensed band provides enough bandwidth to provideQoS to the cellular users at blocks 406-408.

If the SBS needs additional spectrum to serve the cellular users, thesteps 410-416 provide such additional spectrum. In block 410, a targetWiFi user collision probability is provided (e.g., set). The SBSdetermines the fraction of SBS air time S on the unlicensed spectrum forthe cellular user for this probability. Then, in processing block 412,the SBS determines the set of cellular users to be served on the givenlicensed band C and the fraction of the air time share on the unlicensedband S. Thereafter, the SBS estimates the number of WiFi users on theunlicensed spectrum (processing block 414) and determines the minimumbackoff window size W (processing block 416). The sequence processingblock 410-416 may be iteratively repeated.

FIG. 5 shows an adaptive channel access scheme 500 for at least one WiFiAP according to an embodiment. Here, in a first step 502, the SBSinitializes and prepares itself to communicate with cellular users.Then, in the next step, 504, the SBS determines whether or not enoughlicensed band is available to meet QoS for the cellular users. If thereis enough available licensed band C the SBS does not use the unlicensedband(s) but rather only uses the licensed band 506. If there is notenough available licensed band C the SBS determines the possibility toaccess the unlicensed band.

Accordingly, the scheme 500 advances to decision block 508. At decisionblock 508 the SBS listens to the unlicensed band and evaluates whetherthe unlicensed band is idle. If it is idle, the process 500 moves toprocessing block 510. Here, the SBS waits for a distributed inter framespace (DIFS) duration. Then, in the next step 512, the SBS listens againto the unlicensed band and determines whether the unlicensed band isidle. In steps 508-512 the SBS may determine that the unlicensed band isidle for a certain time interval.

If the SBS the unlicensed band is idle, the SBS uses and transmits dataon the unlicensed band for a maximum period of time in process block514. E.g., the maximum period of time may be the transmissionopportunity time (TXOP) as specified in 802.11. The TXOP may comprise amaximum frame length of 3 ms (or 3 subframes). In the next processingstep 516 the SBS may update the average transport block error rate(TBER). When this is done, the SBS updates the minimum backoff windowsize W_(min) in step 518. The update of the minimum backoff window sizemay be performed to other embodiments of the invention, in particular tothe embodiments described in FIGS. 2 and 3. After the minimum backoffwindow size W_(min) is updated the process 500 moves back to thebeginning of the access scheme 500, process block 502. The process 500may iterate once or several times all the process steps described supra.

Returning to decision blocks 508 and 512. When the determination whetherthe unlicensed band is idle is negative, the process 500 advances toprocess step 530. Then the SBS waits until the unlicensed band becomesidle. If the band is idle the SBS ensures to wait for DIFS duration 532.After waiting, the process 500 moves to decision block 534 in order toevaluate whether the unlicensed band is idle and if it is the processprogresses to decision block 534. In the next decision block 536 the SBSdeterminates whether the average TBER (transport block error rate) isabove a threshold. If the SBS determines no the current backoff windowsize is set to the minimum backoff window size W_(min) in processingblock 538. If the SBS determines yes, the current backoff window sizeW_(t) is calculated to be the minimum of 2 times the current backoffwindow size W_(t) and 2 times the maximum backoff window size W_(max) inprocessing block 540.

Then in the next step 550 the backoff window size W is randomly selectedto be in the interval of W=rand (0, W_(t)). In other words, the SBSselects the backoff window size W to be between 0 and the backoff windowsize W_(t). In the next step the process advances to decision block 552.The SBS determines whether the unlicensed band is still idle. If the SBSdetermines no, the SBS repeats this determination until the unlicensedband is not idle anymore. If the unlicensed band is not idle, theprocess advances to decision block 554. Here the SBS evaluates whetherthe backoff window size W is 0. If the backoff window size W is not 0the process moves to process block 556 which may set the backoff windowsize to the backoff window size minus a constant (such as the number 1).If the backoff window size is the backoff window size minus 1 theprocess moves to the input of decision block 552 and goes through therespective steps again. Is however the backoff window size 0 at block554 the SBS moves the process to processing block 514 where the SBS mayuse the unlicensed band for a maximum duration of a TXOP.

FIG. 6 shows the fraction of unlicensed band occupied versus the numberof served cellular users for different licensed spectrum bandwidths whenthe SBS only shares the unlicensed band with one WiFi AP. As can beseen, when the available bandwidth of the licensed spectrum is fixed(e.g., 6 MHz, 8 MHz, 10 MHz, and 12 MHz) the requested unlicensed bandin the SBS will increase as the number of cellular users increases inorder to satisfy the QoS. When the number of cellular users is fixedmore available licensed bandwidth means less utilization of theunlicensed band in the SBS. Accordingly, there may be a tradeoff betweenthe available licensed bandwidth and the required unlicensed bandwidthin the SBS.

When the number of cellular users increases more unlicensed spectrum isrequired to satisfy the QoS for the cellular users. Without implementingaccess control the collision probability brought to the WiFi users onthe unlicensed band increases. This is shown in FIG. 7 for an availablelicensed bandwidth of 8 MHz. As can be seen, by adding cellular users tothe unlicensed band the collision probability to the WiFi usersdramatically increases. By providing an access control scheme for thecellular users the collision probability of the WiFi users iscontrolled. A fair collision probability for these users may beprovided.

FIG. 8 shows the effect of an admission control for the SBS on theunlicensed band according to an embodiment. For up to 11 cellular usersthe achievable licensed bandwidth is enough to satisfy the QoS.Accordingly, the corresponding collision probability to the WiFi usersis not related to and independent from the number of cellular users.However, when the number of cellular users is increased to 12-14 usersthe SBS requires a portion of the unlicensed band to serve all cellularusers. Accordingly, adding additional users has an impact on thecollision probability of the WiFi users and the collision probability ofthe WiFi users increase with the number of the cellular users. However,the collision probability of the WiFi users is below a determined or setthreshold value. All cellular users can be served by the SBS. As is alsoshown in FIG. 8, the threshold value may be passed sooner when there aremore WiFi users compared to when there are fewer WiFi users.

For 16 and more cellular users, the requested air time share on theunlicensed band in the SBS is too large so that access is cut off andonly some of the cellular users can be served to keep the collisionprobability under the threshold value. As discussed before, the air timeshare of the unlicensed band can be adaptively adjusted to provide QoSto the cellular users according to the available licensed bandwidth, theWiFi traffic on the unlicensed band and the number of the cellularusers.

FIG. 9 illustrates results of a throughput on the unlicensed band basedon different access schemes. ACA is the adaptive channel access scheme,NAS is the non-adaptive channel access scheme (where the SBS uses thesame backoff parameters (e.g., minimum backoff window size) as the WiFiusers) and EWS is known as an exclusive WiFi scheme (where the SBS isreplaced by a WiFi AP, i.e., providing an exclusive WiFi network). FIG.9 shows the throughput on the unlicensed band versus the number ofcellular users. The achievable throughput on the unlicensed bandincreases as the number of cellular users increases for ACA and NASwhile the throughput on unlicensed band for EWS does not increase. Thethroughput for the ACA is much better than for NAS because the SBS justtakes enough unlicensed bandwidth to satisfy the QoS of the cellularusers in ACA.

FIG. 10 is a block diagram of a device that may implement embodiments ofthe access scheme disclosed herein. Specific devices may utilize all ofthe components shown, or only a subset of the components, and levels ofintegration may vary from device to device. Furthermore, a device maycontain multiple instances of a component, such as multiple processingunits, processors, memories, transmitters, receivers, etc. The devicemay comprise a processing unit equipped with one or more input/outputunit, such as a speaker, microphone, mouse, touchscreen, keypad,keyboard, printer, display, and the like. The processing unit mayinclude a central processing unit (CPU), memory, a mass storage device,a video adapter, and an I/O interface connected to a bus. The device maybe a base station or a small cell base station.

The bus may be one or more of any type of several bus architecturesincluding a memory bus or memory controller, a peripheral bus, videobus, or the like. The CPU may comprise any type of electronic dataprocessor. The memory may comprise any type of non-transitory systemmemory such as static random access memory (SRAM), dynamic random accessmemory (DRAM), synchronous DRAM (SDRAM), read-only memory (ROM), acombination thereof, or the like. In an embodiment, the memory mayinclude ROM for use at boot-up, and DRAM for program and data storagefor use while executing programs.

The mass storage device may comprise any type of non-transitory storagedevice configured to store data, programs, and other information and tomake the data, programs, and other information accessible via the bus.The mass storage device may comprise, for example, one or more of asolid state drive, hard disk drive, a magnetic disk drive, an opticaldisk drive, or the like.

The video adapter and the I/O interface provide interfaces to coupleexternal input and output devices to the processing unit. Asillustrated, examples of input and output devices include the displaycoupled to the video adapter and the mouse/keyboard/printer coupled tothe I/O interface. Other devices may be coupled to the processing unit,and additional or fewer interface cards may be utilized. For example, aserial interface such as Universal Serial Bus (USB) (not shown) may beused to provide an interface for a printer.

The processing unit also includes one or more network interfaces, whichmay comprise wired links, such as an Ethernet cable or the like, and/orwireless links to access nodes or different networks. The networkinterface allows the processing unit to communicate with remote unitsvia the networks. For example, the network interface may providewireless communication via one or more transmitters/transmit antennasand one or more receivers/receive antennas. In an embodiment, theprocessing unit is coupled to a local-area network or a wide-areanetwork for data processing and communications with remote devices, suchas other processing units, the Internet, remote storage facilities, orthe like.

The processing unit (CPU) may receive the cellular user information andthe WiFi user information through the network interface. The CPU maycalculate, estimate and determine different parameters (such as WiFitraffic, maximum available bandwidth on the unlicensed band, amount ofachievable licensed bandwidth and backoff window size) and may storethese parameter in the mass storage or the memory. The processing unitmay communicate these parameters to the cellular users via the networkinterface.

Embodiments of the invention provide a method for adaptive channelaccess wherein the method comprises adaptively adjusting, by a smallbase station (SBS), access parameters for small cells to ensure qualityof service (QoS) to cellular users while minimizing a collisionprobability to WiFi users, and wherein minimizing the collisionprobability to the WiFi users comprises keeping or decreasing an optimalbackoff window size when the collision probability to the WiFi users isbelow a threshold value and increasing the optimal backoff window sizewhen the collision probability to the WiFi users is above the thresholdvalue.

Embodiments further provide that minimizing the collision probability tothe WiFi users comprises estimating a number of WiFi users on anunlicensed spectrum or wherein minimizing the collision probability tothe WIFI users comprises determining a fraction of SBS air time on anunlicensed spectrum.

Various embodiments provide that the access parameters are adjustedbased on a WiFi traffic load, an achievable licensed bandwidth and QoSrequirements of the cellular users.

Further embodiments provide that minimizing the collision probability tothe WiFi users comprises ensuring QoS requirements for the cellularusers that corresponds to minimum data rates for the cellular users, andwherein the minimum data rates depend on a combined transmit powers ofthe cellular users on a licensed band, a combined allocation of thecellular users on the licensed band, and a combined air time fraction ofthe cellular users on an unlicensed band.

Some embodiments provide that minimizing the collision probability tothe WiFi users comprises setting the optimal backoff window size toinfinity when a licensed spectrum provides enough bandwidth to guaranteeQoS to the cellular users.

Yet other embodiments provide that increasing the optimal backoff windowsize comprises decreasing a fraction of SBS air time share on anunlicensed spectrum.

Embodiments of the invention provide a method for adaptive channelaccess, wherein the method includes determining, by a small base station(SBS), a fraction of SBS air time share on an unlicensed band forcellular users to ensure quality of service (QoS) for the cellularusers, estimating, by the SBS, a number of WiFi users on the unlicensedband, adjusting, by the SBS, the fraction of SBS air time share on theunlicensed band for the cellular users when a collision probability tothe WiFi users is above a threshold value and not adjusting, by the SBS,the fraction of SBS air time share on the unlicensed band for thecellular users when the collision probability to the WiFi users is belowthe threshold value.

Further embodiments provide determining the fraction of SBS air timeshare comprises minimizing the collision probability to the WiFi users.

Other embodiments provide minimizing the collision probability to theWiFi users comprises ensuring QoS requirements for the cellular usersthat corresponds to minimum data rates for the cellular users, andwherein the minimum data rates depend on a combined transmit powers ofthe cellular users on a licensed band, a combined allocation of thecellular users on the licensed band, and a combined air time fraction ofthe cellular users on an unlicensed band.

Yet other embodiments further include keeping or reducing an optimalbackoff window size when the collision probability to the WiFi users isbelow the threshold value and keeping or increasing the optimal backoffwindow size when the collision probability to the WiFi users is abovethe threshold value.

Some embodiments further includes determining, by the SBS, whether ornot an achievable licensed band is enough to ensure the QoS for thecellular users, setting an SBS backoff window size to infinity whenthere is enough achievable licensed band available and setting the SBSbackoff window size to an optimal backoff window size when there is notenough achievable licensed band available.

Various embodiments include adjusting the fraction of SBS air time shareon the unlicensed band for the cellular users comprises reducing thefraction of SBS air time share on the unlicensed band for the cellularusers.

Some embodiments further include determining, by the SBS, an optimal setof the cellular users to be served on a licensed band and the fractionof SBS air time share on the unlicensed band.

Embodiments further include evaluating, by the SBS, whether theunlicensed band is idle, sending data, by the SBS, for a maximumtransmission opportunity duration (TXOP) when the unlicensed band isidle and update an average transport block error rate (TBER).

Embodiments of the invention provide a small base station (SBS)including a processor and a non-transitory computer readable storingmedium storing programming for execution by the processor, theprogramming including instruction to adaptively adjust access parametersfor a small cell to ensure quality of service (QoS) to cellular userswhile minimizing a collision probability to WiFi users, whereinminimizing the collision probability to the WiFi users comprises keep ordecrease an optimal backoff window size when the collision probabilityto the WiFi users is below a threshold value and increase the optimalbackoff window size when the collision probability to the WiFi users isabove the threshold value.

Some embodiments include that the access parameters are adjusted basedon a WiFi traffic load, an achievable licensed bandwidth, and QoSrequirements of the cellular users.

Various embodiments include that minimizing the collision probability tothe WiFi users comprises ensuring QoS requirements for the cellularusers that corresponds to minimum data rates for the cellular users, andwherein the minimum data rates depend on a combined transmit powers ofthe cellular users on a licensed band, a combined allocation of thecellular users on the licensed band, and a combined air time fraction ofthe cellular users on an unlicensed band.

Other embodiments include that increasing the optimal backoff windowsize comprises decreasing a fraction of SBS air time share on an optimalunlicensed band.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. A method for adaptive channel access, the methodcomprising: determining, by a small base station (SBS), a collisionprobability among WiFi users of an unlicensed band based on an estimatednumber of WiFi users and a channel access probability of the SBS usingthe unlicensed band; adjusting, by the SBS, access parameters forcellular users served by the SBS to share air time on the unlicensedband according to quality of service (QoS) requirements for the cellularusers, subject to a comparison between the collision probability amongthe WiFi users of the unlicensed band and a collision probabilitythreshold; determining a fraction of the air time on the unlicensed bandto be used by the cellular users according to the access parameters; anddetermining a maximum number of cellular users that can be admitted tothe SBS according to the fraction of the air time on the unlicensed bandto be used by the cellular users.
 2. The method according to claim 1,wherein adaptively adjusting the access parameters comprises minimizingthe collision probability to the WiFi users, wherein minimizing thecollision probability to the WiFi users comprises: keeping or decreasingan optimal backoff window size when the collision probability to theWiFi users is below a threshold value; and increasing the optimalbackoff window size when the collision probability to the WiFi users isabove the threshold value.
 3. The method according to claim 2, whereinminimizing the collision probability to the WIFI users comprises atleast one of estimating a number of WiFi users on an unlicensed spectrumand determining a fraction of SBS air time on an unlicensed spectrum. 4.The method according to claim 1, wherein the access parameters areadjusted based on a WiFi traffic load, an achievable licensed bandwidthand QoS requirements of the cellular users.
 5. The method according toclaim 1, wherein minimizing the collision probability to the WiFi userscomprises ensuring QoS requirements for the cellular users thatcorresponds to minimum data rates for the cellular users, and whereinthe minimum data rates depend on a combined transmit powers of thecellular users on a licensed band, a combined allocation of the cellularusers on the licensed band, and a combined air time fraction of thecellular users on an unlicensed band.
 6. The method according to claim1, wherein minimizing the collision probability to the WiFi userscomprises setting the optimal backoff window size to infinity when alicensed spectrum provides enough bandwidth to guarantee QoS to thecellular users.
 7. The method according to claim 2, wherein increasingthe optimal backoff window size comprises decreasing a fraction of SBSair time share on an unlicensed spectrum.
 8. A method for adaptivechannel access, the method comprising: determining, by a small basestation (SBS), a fraction of SBS air time share on an unlicensed bandfor cellular users to ensure quality of service (QoS) for the cellularusers; determining a maximum number of cellular users that can beadmitted to the SBS according to the fraction of the air time on theunlicensed band to be used by the cellular users; estimating, by theSBS, a number of WiFi users on the unlicensed band; determining, by theSBS, a collision probability among WiFi users of the unlicensed bandbased on the estimated number of WiFi users and a channel accessprobability of the SBS using the unlicensed band; adjusting, by the SBS,the fraction of SBS air time share on the unlicensed band for thecellular users when the collision probability to the WiFi users is abovea threshold value; and not adjusting, by the SBS, the fraction of SBSair time share on the unlicensed band for the cellular users when thecollision probability to the WiFi users is below the threshold value. 9.The method according to claim 8, wherein determining the fraction of SBSair time share comprises minimizing the collision probability to theWiFi users.
 10. The method according to claim 9, wherein minimizing thecollision probability to the WiFi users comprises ensuring QoSrequirements for the cellular users that corresponds to minimum datarates for the cellular users, and wherein the minimum data rates dependon a combined transmit powers of the cellular users on a licensed band,a combined allocation of the cellular users on the licensed band, and acombined air time fraction of the cellular users on an unlicensed band.11. The method according to claim 8, further comprising: keeping orreducing an optimal backoff window size when the collision probabilityto the WiFi users is below the threshold value; and keeping orincreasing the optimal backoff window size when the collisionprobability to the WiFi users is above the threshold value.
 12. Themethod according to claim 8, further comprising: determining, by theSBS, whether or not an achievable licensed band is enough to ensure theQoS for the cellular users; setting an SBS backoff window size toinfinity when there is enough achievable licensed band available; andsetting the SBS backoff window size to an optimal backoff window sizewhen there is not enough achievable licensed band available.
 13. Themethod according to claim 8, wherein adjusting the fraction of SBS airtime share on the unlicensed band for the cellular users comprisesreducing the fraction of SBS air time share on the unlicensed band forthe cellular users.
 14. The method according to claim 13, furthercomprising determining, by the SBS, an optimal set of the cellular usersto be served on a licensed band and the fraction of SBS air time shareon the unlicensed band.
 15. The method according to claim 8, furthercomprising: evaluating, by the SBS, whether the unlicensed band is idle;sending data, by the SBS, for a maximum transmission opportunityduration (TXOP) when the unlicensed band is idle; and update an averagetransport block error rate (TBER).
 16. A small base station (SBS)comprising: a processor; and a non-transitory computer readable storingmedium storing programming for execution by the processor, theprogramming including instruction to: determining, by a small basestation (SBS), a collision probability to WiFi users of an unlicensedband based on an estimated number of WiFi users and a channel accessprobability of the SBS using the unlicensed band; adaptively adjustaccess parameters for cellular users served by the SBS to share air timeon an unlicensed band according to quality of service (QoS) requirementsfor the cellular users, subject to a comparison between the collisionprobability to WiFi users of the unlicensed band and a collisionprobability threshold; determine a fraction of the air time on theunlicensed band to be used by the cellular users according to the accessparameters; and determining a maximum number of cellular users that canbe admitted to the SBS according to the fraction of the air time on theunlicensed band to be used by the cellular users.
 17. The small basestation according to claim 16, wherein the instructions to adaptivelyadjust the access parameters comprises instructions to minimize thecollision probability to the WiFi users, wherein the instructions tominimize the collision probability to the WiFi users comprisesinstructions to: keep or decrease an optimal backoff window size whenthe collision probability to the WiFi users is below a threshold value;and increase the optimal backoff window size when the collisionprobability to the WiFi users is above the threshold value.
 18. Thesmall base station according to claim 17, wherein the instructions tominimize the collision probability to the WiFi users compriseinstructions to ensure QoS requirements for the cellular users thatcorrespond to minimum data rates for the cellular users, and wherein theminimum data rates depend on a combined transmit powers of the cellularusers on a licensed band, a combined allocation of the cellular users onthe licensed band, and a combined air time fraction of the cellularusers on an unlicensed band.
 19. The small base station according toclaim 17, wherein increasing the optimal backoff window size comprisesdecreasing a fraction of SBS air time share on an optimal unlicensedband.
 20. A small base station (SBS) comprising: a processor; and anon-transitory computer readable storage medium storing programming forexecution by the processor, the programming including instructions for:initializing a maximum fraction of SBS air time shared on an unlicensedspectrum according to a WiFi traffic estimate; determining that anamount of available licensed spectrum does not guarantee a quality ofservice (QoS) to small cell users; determining power and rateoptimization to minimize a collision probability of WiFi users; anddetermining an SBS backoff window size according to the maximum fractionof SBS air time, the WiFi traffic estimate, the QoS, the poweroptimization, and the rate optimization.